This invention generally relates to electronic systems and in particular it relates to pass device leakage current correction circuits for use in linear regulators.
Because of the large physical size of the pass element that is used in linear regulators it can have a large amount of off-state leakage current especially at high temperatures. When no load is applied to the output this leakage current can cause the output to go out of regulation. And in fact may cause the output to rail to the input supply if the leakage current is significant. In the past, one possible solution was to simply require a minimum load current on the output; however, this is not a feasible solution for applications that operate in no-load or low-load conditions. Another possible solution is the use of a constant internal pull-down on the output. The problem with this technique is that the quiescent current significantly increases in order to provide this constant pull-down. Finally, one last alternative is the use of a current comparator. This circuit essentially compares the two current legs in the main differential amplifier. Under steady-state condition when the output is in regulation these currents should be equal. However, when the pass element leaks and the output goes out of regulation these currents are no longer equal. A simple current comparator can be used to gain up this differential current and apply it to the output.
The leakage current correction circuit uses a dummy device to detect leakage current. The dummy device is a scaled down version of the pass element. A current router is used to either gain up the dummy leakage current and apply it to the output or simply dump the small un-gained current to ground.